Josephson Clinical Cardiac Electrophysiology

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■ Josephson’s Clinical Cardiac Electrophysiology

CL 700 I II III

A

A

A

A

HRA A

AH V

V

H

V

H

V

A

AH

A

A

HBE

T

FIGURE 5.20 Bradycardia-dependent bundle branch block. The high right atrium (HRA) is paced at a cycle length (CL) of 700 msec. Type I second-degree block in the atrioventricular (A-V) node is present, as evidenced by the lack of a His bundle deflection following the fourth atrial complex and a shortening of the A-H interval or of the next conducted impulse compared with the preceding complexes. The conducted atrial complex following the pause manifests a left bundle branch block configuration, as does the first complex (which also followed a pause). Because the block is occurring in the A-V node, Phase 4 depolarization can begin in the left bundle branch in such a way that the next conducted beat is propagated slowly (or not at all) through this structure (see text).

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